I like 17s. I will not go any bigger than 17. Tires are cheaper and the ride is smooth. They may not look agressive as 18s.

 

18"s all the way

 

17 unless you are planning to run a big brake kit and that is serious cash ~$4K and up. I hate the gap between big wheels and small brakes. The cost of 18 is much higher, fuel economy, new tires, etc. Unless it is a dedicated car, i.e. track, show or Sunday then 17 is much easier to live with. And if you ever need it, try getting snow tires or chains on an 18.